In nuclear magnetic resonance (NMR) spectrometers, it is desired to produce a homogeneous field in a sample to be analyzed. Typically, the sample is located in a vial that is inserted into a magnetic field desirably having uniform characteristics throughout the length of the sample. In some spectrometers, the magnetic field is provided by a superconducting, solenoid coil having an interior into which the vial and sample are placed. Numerous winding arrangements have been employed in attempts to enable the field within the interior of the solenoid to be uniform along the solenoid longitudinal axis.
Exemplary of the prior art showing apparatus for enabling substantially homogeneous magnetic fields to be produced in NMR superconducting, solenoid coils are the commonly assigned U.S. Pat. Nos. to Gang 3,287,630, Werner et al 3,419,904, Nelson 3,564,398 and Werner 3,577,067. In the Gang patent, correction for first, second and third order axial field gradients is obtained by utilizing correcting shim coils that are mounted in proximity to the exterior cylindrical surface of a solenoid, in combination with a coil assembly mounted inside of the main solenoid winding in very close proximity to the sample holding vial. The coil assembly includes four groups of coaxially aligned coils that provide first order, second and third order corrections for linear gradients of the main coil. To compensate for variations of the magnetic field of the main coil relative to the transverse axis of that coil, there are provided circular coils having longitudinal axes aligned with the transverse axes of the main coil. Each set of coils in this prior art device is supplied with a separate, independently controlled current, so that the different coil sets are generally energized with differing currents from a DC power supply source.
In the coil disclosed in the Nelson patent, a plurality of correcting coils are longitudinally positioned at different regions inside of the main coil so that the individual correcting coils have spatial independence. In the Werner and Weaver et al patents, as in the Nelson patent, different currents are supplied to the individual coils to achieve the desired corrections.
To facilitate energization of superconducting coils, it is desired for all of the coil segments to be supplied with the same series current. It is difficult to supply different windings or winding portions of the coil with different currents in a superconducting magnet system because the coil is in a relatively inaccessible position relative to the power supply, i.e., in the interior of a Dewar maintained at a cryogenic temperature on the order of 4.2.degree. K. Also, it is desirable to minimize the number of leads connecting the coil to the environment outside of the Dewar to reduce the coupling of heat from outside of the Dewar to the coil.
Other disadvantages were associated with the prior art structures. In particular, they usually required empirical studies to determine the correct current to be supplied to each of the coils. While some of the prior art coils corrected for inhomogeneities of the longitudinal magnetic field, through fifth order axial gradients, it is necessary for certain purposes, to correct for axial gradients as high as the seventh order.